Method for sealing a quartz crystal device

ABSTRACT

A quartz crystal device includes a crystal resonator element and a package including a plurality of components. The plurality of components are bonded using a metal paste sealing material containing a metallic particle having an average particle size from 0.1 to 1.0 μm, an organic solvent, and a resin material in proportions of from 88 to 93 percent by weight from 5 to 15 percent by weight, and from 0.01 to 4.0 percent by weight, respectively, to hermetically seal the crystal resonator element in the package.

This application claims priority from Japanese Patent Application No.2007-073824 filed in the Japanese Patent Office on Mar. 22, 2007, andfrom Japanese Patent Application No. 2007-286419 filed in the JapanesePatent Office on Nov. 2, 2007, the entire disclosures of which arehereby incorporated by reference in their entireties.

BACKGROUND

1. Technical Field

The present invention relates to a metal paste sealing material forhermetically sealing, e.g., a tuning fork type or thickness shearvibration mode crystal resonator element in a package in a quartzcrystal device such as a crystal unit, a crystal resonator, a crystaloscillator, a crystal filter, or a crystal sensor, and a quartz crystaldevice sealed using the metal paste sealing material and a method forsealing the quartz crystal device.

2. Related Art

Demands for smaller, thinner piezoelectric devices have been increasing,and many surface mount piezoelectric devices, which are suitable formounting onto circuit boards and the like, have hitherto been used.

In surface mount piezoelectric devices, a configuration is employed inwhich a piezoelectric resonator element is sealed in a package made ofan insulating material such as ceramic.

With a package in this former configuration, a piezoelectric resonatorelement is mounted in a cavity where thin plates made of ceramicmaterials are laminated on a box base, and a lid is hermetically bondedto the base so that the piezoelectric resonator element is sealed.

Regarding bonding of a base and a lid, in the case of a lid made ofmetal, a method of placing a metal seal ring on the top end surface ofthe base and seam-welding the lid to the seal ring (e.g.,JP-A-2000-223606) and a method of placing a brazing metal material onthe top end surface covered with a metal film of a base and heating andmelting the material (see, e.g., JP-A-11-307661) are known.

In the case of a lid made of a ceramic or glass material, a method ofheating and melting low-melting glass or resin placed between a base anda lid (see, e.g., JP-A-2001-244772) and a method of forming a metallizedlayer on the lower surface of a lid and melting an eutectic solder sothat the lid is bonded to a ceramic case (see, e.g., JP-A-9-36690), anda method of heating and melting an eutectic metallic film layer formedin a bonding portion of a lid (see, e.g., JP-A-54-78694) are known.

However, there is a possibility that a gas produced from low-meltingglass and high heat of seam-welding decrease or degrade frequencycharacteristics of a crystal resonator element.

Low-melting glass contains lead in many cases, and therefore may be notdesirable in terms of a possibility of adversely affecting theenvironment.

To address these points, there has been proposed a crystal unit with astructure having a reduced size and a thinner thickness, in which acrystal plate having a crystal resonator element and an outer frameintegrated therein is formed and substrates serving as a base and acover are bonded on and under the crystal plate.

To hermetically seal such a structure, for example, a method ofanode-bonding of metallic layers, which are formed on the upper andlower surfaces of an outer frame integrated with a crystal unit, with acover and a case made of glass is known (see, e.g., JP-A-2000-68780).

A method in which surfaces to be mutually bonded of a piezoelectricplate and a substrate that have been subjected to mirror polishing arecleaned by applying ultraviolet rays or oxygen plasma, and the surfacesare bonded together using hydrogen bonding of —OH base by absorbingmoisture is also known (see, e.g., JP-A-7-154177).

A method of putting together a bond surface having an Au film thereon ofa piezoelectric substrate and a bond surface having an Ag film formedthereon of a protective substrate after plasma treatment and applyingpressure and heating the surfaces to achieve diffusion bonding is alsoknown (see, e.g., International Patent Publication No. WO 00/76066pamphlet).

Further, a surface acoustic wave device formed by applying pressure to ametallic film formed on the main surface of a piezoelectric substratehaving an interdigital transducer (IDT) formed thereon and a metallicfilm formed on the main surface of a base substrate, after activatingthe surfaces by ion beam and plasma irradiation, to directly bondtogether the surfaces is known (see, e.g., JP-A-54-78694).

Recently, there have been proposed methods that, in a container tocontain electronic components such as piezoelectric resonators,hermetically seal a bonding portion of a box-type container formed of aceramic material and a cover made of metal using a paste-like sealingmember composed of metallic fine particles having an average particlesize of 1 to 100 nm and a dispersion material such as amine (see, e.g.,JP-A-2005-317793 and JP-A-2005-31-7794).

A method described in JP-A-2005-317793 applies a sealing member to acontainer or a cover by screen printing or an inkjet method and heatingboth the container and the cover in an atmosphere of 250° C. or less tobond together them.

A method described in JP-A-2005-317794 makes a paste-like sealing memberbecome a sheet-like one and, using this one as a cover, bonds togetherthe cover and the container to achieve a hermetic seal.

A piezoelectric device such as a crystal unit is also known.

In this device, a crystal plate are bonded with upper and lower plateswith a metallic fine particle paste bonding material containing metallicfine particles that has an average particle size of 1 to 100 nm, thesurfaces of which are coated with organic films, and which are dispersedin an organic solvent (see, e.g., JP-A-2006-186748).

The paste bonding material hermetically seals a piezoelectric device bymetal-to-metal bonding between metallic fine particles and betweenmetallic films that melts the organic films by heating at about 200° C.to make the metallic fine particles melt with each other, that is, tosinter them.

However, in a pasty sealing member or bond material made of theabove-described metallic fine particles, nanoparticles having an averageparticle size of 100 nm or less are used as the metallic fine particles.

To prevent aggregation of the nanoparticles, a relatively large amountof organic solvent is required as the dispersing agent.

Therefore, gas may be generated from the organic solvent during sealingand remain inside a package, or gas may be generated over time from theorganic solvent remaining in a bonding portion, causing a possibility ofdeteriorating the quality and frequency characteristics of apiezoelectric resonator element.

In general, organic solvent is difficult to handle, and therefore oneproblem with a sealing member containing a large amount of organicsolvent is that the sealing member is not suitable for mass productionand requires careful handling for its storage and transportation.

SUMMARY

An advantage of the invention is to provide a method for sealing apiezoelectric device by which gas cannot be generated from organicsolvent during bonding and after sealing to adversely affect apiezoelectric resonator element and a package can be hermetically sealedwith sufficient bonding strength and high reliability.

Another advantage of the invention is to provide a metal paste sealingmaterial that is used for such a method for sealing a piezoelectricdevice and is relatively easy to handle and suitable for massproduction.

According to a first aspect of the invention, there is provided a quartzcrystal device that includes a crystal resonator element; and a packageincluding a plurality of components.

The plurality of components are bonded using a metal paste sealingmaterial that is made of one kind or two or more kinds of Au, Ag, Pt,and Pd and that contains a metallic particle having an average particlesize from 0.1 to 1.0 μm, an organic solvent, and a resin material inproportions of from 88 to 93 percent by weight, from 5 to 15 percent byweight, and from 0.01 to 4.0 percent by weight, respectively, tohermetically seal the crystal resonator element in the package.

The metal paste sealing material for use in a quartz crystal deviceaccording to the first aspect of the invention has a larger particlesize of the metallic particles and a significantly smaller proportion oforganic solvent than a metallic paste using nano-micron particles of aformer technique.

Therefore, if this metal paste sealing material is sintered atrelatively low temperatures, e.g., from about 200 to about 300° C., itis easily possible to form a sintered body of a porous structure inwhich metallic particles are densified and to sufficiently evaporate anorganic solvent and a resin material so that they are substantially notleft in the sintered body.

Regarding the sintered body of a porous structure, particularly in thecase where the Young's modulus is from 9 to 16 GPa and the density isfrom 10 to 17 g/cm³, if a relatively low pressure is applied to thesintered body, the metallic particles can be more denselyrecrystallized.

This recrystallization allows high air tightness of the package of aquartz crystal device to be secured.

Further, the metal paste sealing material has moderate viscosity bycontaining a minute amount of resin material.

The metal paste sealing material can therefore be applied to a finepattern using publicly known techniques such as screen printing,dispenser, and inkjet, and does not flow out even after the applicationso that the pattern can be maintained.

Such good shapability is similarly effective after sintering at lowtemperatures.

Therefore, when recrystallization has been performed by applyingpressure to a sintered body, the same pattern is maintained as thatduring the application of the metal paste sealing material.

Accordingly, when a package of a quartz crystal device is sealed, thesealing width can be set narrower.

Just by the narrowed amount, the dimensions of the package can bereduced to reduce the entire size of the device, or the internal volumeof the package can be increased while the outside dimensions ismaintained.

In this case, in a quartz crystal device having a such package structurethat the plurality of components include an intermediate crystal platein which the crystal resonator element and an outer frame are integrallyconnected, and an upper substrate and a lower substrate bonded withupper and lower surfaces of the intermediate crystal plate,respectively, with the metal paste sealing material, the upper substratemay be made of a silicon material on which an integrated circuit fordriving the crystal resonator element is formed, and have a terminalcoupled to the integrated circuit on the lower surface; the intermediatecrystal plate may have a terminal at a position corresponding to theterminal of the upper substrate on the upper surface of the outer flame;and the terminal of the upper substrate may be directly coupled with theterminal of the intermediate crystal plate with a conductive couplingmaterial.

Therefore, a smaller, lower crystal oscillator can be achieved.

In this case, in a quartz crystal device having such a package structurethat the plurality of components include a base, formed in a box shapehaving an open top, having the crystal resonator element mounted in aninside thereof, and a lid hermetically bonded onto a top end surface ofthe base with the metal paste sealing material, the lid may be made of asilicon material on which an integrated circuit for driving the crystalresonator element is formed, and have a terminal coupled to theintegrated circuit on a lower surface of the lid; the base may have aterminal at a position corresponding to the terminal of the lid on thetop end surface of the base; and the terminal of the lid may be directlycoupled with the terminal of the base with a conductive couplingmaterial.

Therefore, a smaller, lower crystal oscillator can similarly beachieved.

Thus, a method for hermetically sealing piezoelectric devices made,e.g., of quartz crystal having various package structures utilizing ametal paste sealing material of one aspect of the invention.

According to a second aspect of the invention, there is provided amethod for sealing a quartz crystal device, in order to bond an uppersubstrate and a lower substrate with upper and lower surfaces,respectively, of an intermediate crystal plate in which a crystalresonator element and an outer frame are integrally connected tohermetically seal the crystal resonator element in a cavity definedbetween the upper substrate and the lower substrate.

The method includes:

(a) having a metallic thin film on each of upper and lower surfaces ofthe outer frame of the intermediate crystal plate, and a metallic thinfilm on a surface to be bonded with the outer frame of each of the uppersubstrate and the lower substrate;

(b) applying the above-described metal paste sealing material of oneaspect of the invention to at least one of the metallic thin film on theupper surface of the outer frame and the metallic thin film of the uppersubstrate, and heating the metal paste sealing material to form aprimary sintered body having a porous structure with a Young's modulusof 9 to 16 GPa and a density of 10 to 17 g/cm³;

(c) similarly applying the above-described metal paste sealing materialof one aspect of the invention to at least one of the metallic thin filmon the lower surface of the outer frame and the metallic thin film ofthe lower substrate, and heating the metal paste sealing material toform a primary sintered body having a porous structure with a Young'smodulus of 9 to 16 GPa and a density of 10 to 17 g/cm³;

(d) placing the intermediate crystal plate and the upper substrate ontop of each other while bringing one of the metallic thin films havingthe primary sintered bodies formed thereon into contact with the otherof the metallic thin films, and applying pressure to the primarysintered bodies to densely recrystallize metallic particles thereof tohermetically bond the intermediate crystal plate and the upper substratein the outer frame; and

(e) placing the intermediate crystal plate and the lower substrate ontop of each other while bringing one of the metallic thin films havingthe primary sintered bodies formed thereon into contact with the otherof the metallic thin films, and applying pressure to the primarysintered bodies to densely recrystallize metallic particles thereof tohermetically bond the intermediate crystal plate and the lower substratein the outer frame.

As described above, a metal paste sealing material of one aspect of theinvention keeps sufficient and good shapability not only during itsapplication but also after the steps (b) to (e), and has high airtightness after the steps (d) and (e) due to recrystallization of themetallic particles.

This enables the sealing width in the outer frame to be madesignificantly narrower than a former one.

Therefore, it is possible to reduce the outside dimensions of theintermediate crystal plate to reduce the entire size of the quartzcrystal device.

Alternatively, it is possible to increase the inside dimensions whilemaintaining the outer dimensions of the outer frame, and increase, byjust the amount of the increase of the inside dimensions, the outsidedimensions of a crystal resonator element that can be mounted, therebyimproving the characteristics of a quartz crystal device.

The primary sintered body having the above-described Young's modulus anddensity is not in the original paste state but has some extent ofsoftness.

Accordingly, in placing the intermediate crystal plate and each of theupper substrate and the lower substrate on top of each other andapplying pressure to them in the steps (d) and (e), there is nopossibility that the material flies to adhere to the crystal resonatorelement or to be left inside the device.

Therefore, the vibration characteristics of the quartz crystal device ismaintained without being deteriorated.

The porous structure of a primary sintered body can be formed byperforming the steps (b) and (c) at relatively low temperatures.

Therefore, if the steps (b) and (c) are performed by applying the metalpaste sealing material to the intermediate crystal plate, the thermalstress applied to the crystal resonator element can be suppressed.

Further, since the organic solvent and resin material of the metal pastesealing material are substantially not left in the primary sinteringbody, outer gas of the organic solvent or the resin is not generatedfrom a primary sintered body, and is not left inside the piezoelectricdevice in the steps (d) and (e).

Further, outer gas of the organic solvent or resin cannot besequentially generated.

Therefore, good quality and frequency characteristics of the quartzcrystal device can be maintained for a long period.

Regarding a quartz crystal device having this package structure, theupper substrate and the lower substrate may be made of the same quartzcrystal as of the intermediate crystal plate.

Accordingly, the entire package has the same coefficient of thermalexpansion.

This suppresses distortion caused by changes in environmentaltemperature, etc., which improves reliability.

In this case, the upper substrate may be made of a silicon material andinclude an integrated circuit for driving the crystal resonator elementand a terminal coupled to the integrated circuit; the intermediatecrystal plate may have a terminal to be coupled to the terminal of theupper substrate on the upper surface of the outer frame; and in the step(d), in placing, the intermediate crystal plate and the upper substrateon top of each other, the terminal of the upper substrate may bedirectly coupled with the terminal of the intermediate crystal platewith a conductive coupling material.

Thus, a quartz crystal device functioning as an oscillator can bemanufactured with a relatively small number of man-hours.

According to a third aspect of the invention, there is provided a methodfor sealing a quartz crystal device, in order to bond a lid with a topend surface of a base that is formed in a box shape having an open topand has a crystal resonator element mounted in an inside thereof toachieve a hermetic seal.

The method includes:

(a) having a metallic surface on the top end surface of the base, and ametallic surface on a surface of the lid to be bonded with the top endsurface;

(b) applying the metal paste sealing material of one aspect of theinvention to at least one of the metallic surface on the top end surfaceof the base and the metallic surface of the lid, and heating the metalpaste sealing material to form a primary sintered body having a porousstructure with a Young's modulus of 9 to 16 GPa and a density of 10 to17 g/cm³; and

(c) placing the lid on top of the base while bringing one of themetallic surfaces having the primary sintered bodies formed thereon intocontact with the other of the metallic surfaces, and applying pressureto the primary sintered bodies to densely recrystallize metallicparticles thereof to achieve a hermetic seal.

In a quartz crystal device having such a package structure, theexcellent operation and effect of the above-described sealing methodaccording to one aspect of the invention are similarly obtained.

In particular, unlike former methods of bonding the base and the lidwith a brazing metal material and by seam-welding, the effect of thermalstress to a piezoelectric resonator element mounted on the base can besuppressed.

Since the method involves no danger that a sealing material flies toadhere to a crystal resonator element as in the former seam-welding, thevibration characteristics of a quartz crystal device can be maintainedwithout being deteriorated.

Further, since a bonding portion where the metallic particles arerecrystallized by the step (c) cannot be remelted in a high-temperatureenvironment as in the former bonding using a brazing metal material,high air tightness is ensured over time.

Therefore, if a quartz crystal device is heated in a repair process of afaulty part after being mounted on a circuit board or the like, airtightness cannot be reduced, allowing high reliability to be maintained.

In a piezoelectric device, e.g., of quartz crystal having this packagestructure, the lid may be made of a glass plate, and have a metallicfilm formed on one surface thereof.

This can provide a metallic surface for being bonded with a primarysintered body on the top end surface of the base or for application of ametal paste sealing material.

In this case, a metallic film is not formed in some area of the lowersurface of the lid, allowing the frequency of the crystal resonatorelement to be adjusted after sealing by irradiation of laser lighttransmitting the lid from the outside.

In this case, the lid may be made of a metallic plate and one surfacethereof can be bonded with a primary sintered body on the top endsurface of the base without the application of a material, or can beutilized as a metallic surface for applying a metal paste sealingmaterial.

This is advantageous because formation of a metallic film is thereforeunnecessary.

In this case, the lid may be made of a silicon material and includes anintegrated circuit for driving the crystal resonator element and aterminal coupled to the integrated circuit; the base may have a terminalto be coupled to the terminal of the lid on the top end surface of thebase; and in the step (c), in placing the lid on top of the base, theterminal of the lid may be directly coupled with the terminal of thebase with a conductive coupling material.

Thus, a quartz crystal device functioning as an oscillator can bemanufactured with a relatively small number of man-hours.

In this case, in the steps (d) and (e), heating may be simultaneous withapplying pressure.

Thus, bonding of metallic particles of the primary sintered body byrecrystallization can be made better and effectively.

Further, according to a fourth aspect of the invention, there isprovided a method for sealing an electronic component, in an electroniccomponent having a package that includes a first component and a secondcomponent, in order to bond the first component with the secondcomponent to hermetically seal an electronic element or the like in acavity defined between the first component and the second component.

The method includes:

(a) having metallic surfaces on surfaces to be bonded with each other ofthe first component and the second component;

(b) applying a metal paste sealing material of one aspect of theinvention to at least one of the metallic surfaces of the firstcomponent and the second component, and heating the metal paste sealingmaterial to form a primary sintered body having a porous structure witha Young's modulus of 9 to 16 GPa and a density of 10 to 17 g/cm³; and

(c) placing one and the other of the metallic surfaces having theprimary sintered bodies formed thereon on top of each other whilebringing them into contact with each other, and applying pressure to theprimary sintered bodies to densely recrystallize metallic particlesthereof to achieve a hermetic seal.

various electronic components other than piezoelectric devices can behermetically sealed in packages using a metal paste sealing material ofone aspect of the invention.

In the same way, the above-described excellent operation and effect of amethod for sealing a quartz crystal device according to one aspect ofthe invention can be obtained.

In this case, by performing heating simultaneously with applyingpressure in the step (c), bonding of metallic particles of the primarysintered body by recrystallization can be made better and effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described in detail belowwith reference to the accompanying drawings.

FIGS. 1A and 1B are sectional views illustrating a process of sealing acrystal unit of a first embodiment according to a method of oneembodiment of the invention;

FIG. 2A is a top view of an intermediate crystal plate constituting thecrystal unit of the first embodiment; FIG. 2B is a bottom view of theintermediate crystal plate; and FIG. 2C is a sectional view of theintermediate crystal plate;

FIG. 3A is a bottom view of an upper substrate constituting the crystalunit of the first embodiment; and FIG. 3B is a sectional view of theupper substrate;

FIG. 4A is a top view of a lower substrate constituting the crystal unitof the first embodiment; and FIG. 4B is a sectional view of the lowersubstrate;

FIGS. 5A to 5C are partially enlarged sectional views schematicallyillustrating a process of a metal paste sealing material from its pastestate to recrystallization in the process of sealing of FIGS. 1A and 1B;

FIG. 6A is a schematic perspective view illustrating three crystalwafers to be integrally laminated to one another; and FIG. 6B is apartially enlarged sectional view illustrating an intermediate crystalwafer to which a metal paste sealing material is applied;

FIG. 7A is an explanatory view illustrating procedures for bonding threecrystal wafers; and FIG. 7B is a schematic perspective view illustratinga bonded body of three crystal wafers;

FIG. 8 is a partially enlarged plan view illustrating the intermediatecrystal wafer of FIG. 6 to which a metallic paste material is applied;

FIG. 9A is a sectional view illustrating a process of sealing a crystalunit of a second embodiment by a method of one embodiment of theinvention; and FIG. 9B is a sectional view illustrating the sealedcrystal unit of the second embodiment;

FIG. 10 is a plan view of a base of the crystal unit of the secondembodiment;

FIG. 11 is a bottom view of a lid of the crystal unit of the secondembodiment;

FIG. 12 is a sectional view of a crystal oscillator to which oneembodiment of the invention is applied;

FIG. 13 is a plan view of a base of the crystal oscillator of FIG. 12,

FIG. 14 is a bottom view of a lid of the crystal oscillator of FIG. 12;

FIGS. 15A and 15B are partially enlarged sectional views illustrating asealing process of the crystal oscillator of FIG. 12;

FIG. 16 is a sectional view of another crystal oscillator to which oneembodiment of the invention is applied;

FIG. 17 is a top view of an intermediate crystal plate of the crystaloscillator of FIG. 16;

FIG. 18 is a bottom view of an upper substrate of the crystal oscillatorof FIG. 16;

FIG. 19 is a sectional view of a tuning fork crystal unit to which oneembodiment of the invention is applied;

FIG. 20A is a top view of an intermediate crystal plate of the crystalunit of FIG. 19, and FIG. 20B is a bottom view of the intermediatecrystal plate;

FIG. 21 is a bottom view of an upper substrate of the crystal unit ofFIG. 19; and

FIG. 22 is a top view of a lower substrate of the crystal unit of FIG.19.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described.

First, a metal paste sealing material according to one embodiment of theinvention is composed of metallic particles, an organic solvent, and aresin material.

The metallic particles are submicron particles having an averageparticle size of 0.1 to 1.0 μm made, e.g., of one kind or two or morekinds of Au, Ag, Pt, and Pd.

As the organic solvent, ester alcohol, etc., that can evaporate at arelatively low temperature of about 200° C. are used.

The resin material is added to provide some extent of viscosity to themetal paste sealing material.

For example, a cellulosic resin material is used as the resin material.

Their proportions in the metal paste sealing material are set such thatthe metallic powder is form 88 to 93 percent by weight, the organicsolvent is from 5 to 15 percent by weight, and the resin material isfrom 0.01 to 4.0 percent by weight.

A metal paste sealing material of one embodiment of the invention islarger in size of a metallic particle and significantly smaller inpercentage of an organic solvent than a metal paste using nano-micronsize particles of the former art.

Therefore, sintering this material at relatively low temperatures, e.g.,from about 200 to about 300° C. allows easy formation of a sintered bodyof a porous structure where metallic particles are melt bonded to bedensified, which will be described later.

Moreover, even sintering at low temperatures allows an organic solventand a resin material in a metal paste sealing material to besufficiently evaporated such that they are substantially not left in thesintered body.

In the sintered body of a porous structure, it is easy to make themetallic particles more dense to recrystallize them even by applyingrelatively low pressure.

High air tightness can be secured by this recrystallization, which issuitable particularly for vacuum sealing of packages of piezoelectricdevices and electronic components.

Further, a metal paste sealing material of one embodiment of theinvention contains a minute amount of resin material, and therefore hassuch moderate viscosity that the metal paste sealing material can beeasily applied to a desired, fine pattern using known techniques such asscreen printing, dispenser, and inkjet and, after the application, thepattern can be maintained without the material flowing out.

Such good shapability is the same even after the metal paste sealingmaterial is sintered at low temperatures.

When recrystallization has been performed by applying pressure to asintered body, the same pattern is maintained as that during theapplication of the metal paste sealing material.

Accordingly, when sealing a package of a piezoelectric device or thelike, the sealing width can be set narrower.

FIGS. 1A and 1B illustrate a process of sealing a crystal unit of afirst embodiment by a method of one embodiment of the invention.

A crystal unit 1 of the present embodiment includes a intermediatecrystal plate 2 having a crystal resonator element, an upper substrate 3serving as a cover of a package, and a lower substrate 4 serving as abase.

The intermediate crystal plate 2 is formed of an AT cut crystal plate,and the upper and lower substrates 3 and 4 are laminated on and tinderthe intermediate crystal plate 2 such that the substrates and the plateare integrally bonded.

The upper and lower substrates 3 and 4 are preferably formed of crystalthin plates in the same manner.

Alternatively, they may be formed of a glass material and silicon.

The intermediate crystal plate 2 is made of a crystal thin plate havinga uniform thickness as a whole, and has a thickness shear mode crystalresonator element 5 and an outer frame 6 integrally connected theretowith a base portion 5 a of the element, as illustrated in FIGS. 2A to2C.

Formed on upper and lower surfaces of the crystal resonator element 5are excitation electrodes 7 and 8, respectively.

The excitation electrodes 7 and 8 are led out from the base portion 5 athrough wiring films 7 a and 8 a, and electrically coupled to conductivemetallic thin films 9 and 10 that are formed over all the peripheries ofthe upper and lower surfaces of the outer frame 6, respectively.

A through hole 11 is disposed at an end in the longitudinal direction ofthe outer frame 6 on a side where the crystal resonator element 5 isconnected.

Formed on the lower surface of the foregoing end in the longitudinaldirection is a conductive metallic thin film 13 separated from theconductive metallic thin film 10 by a crystal elemental area 12.

The conductive metallic thin film 13 is electrically coupled with theconductive metallic thin film 9 on the upper surface through aconductive film inside the through hole 11.

In the upper substrate 3, a recess 3 a is formed in a surface facing theintermediate crystal plate 2, that is, the lower surface, as illustratedin FIGS. 3A and 3B.

A periphery surrounding the recess 3 a in the lower surface of the uppersubstrate 3 constitutes a surface to be bonded with the intermediatecrystal plate 2, and is coated with a metallic thin film 14, whichcorresponds to the conductive metallic thin film 9 of the aforementionedouter frame.

In the lower substrate 4, a recess 4 a is similarly formed in a surfacefacing the intermediate crystal plate 2, that is, the upper surface, asillustrated in FIGS. 4A and 4B.

A periphery surrounding the recess 4 a in the upper surface of the lowersubstrate 4 constitutes a surface to be bonded with the intermediatecrystal plate 2, and a metallic thin film 15 that corresponds to theconductive metallic thin film 10 of the aforementioned outer frame and ametallic thin film 17 that is separated from the foregoing metallic thinfilm by a crystal elemental area 16 and corresponds to the conductivemetallic thin film 13 of the outer frame.

Formed on the lower surface of the lower substrate 4 is a leadingelectrode (not illustrated) to the outside.

The conductive metallic thin films 9, 10, and 13 of the intermediatecrystal plate and the metallic thin films 14, 15, and 17 of the upperand lower substrates are preferably formed by laminating an Au film,e.g., on a Cr film, a Ni film, a Ni/Cr film, a Ti film or a Ni—Cr filmas a base film.

In another embodiment, these metallic thin films can be formed by Al,Ag, Cu, Pd, Pt, Sn, Ti, Al/Si, or Ni/Cr.

These metallic thin films are easily formed using methods that arepublicly known, such as sputtering, evaporation, plating, and directplating, alone or in combination.

According to a method of one embodiment of the invention, theabove-described metal paste sealing materials of one embodiment of theinvention, which are denoted by reference numerals 18, 19, and 20, areapplied onto the conductive metallic thin films 9, 10, and 13 on theupper and lower surfaces of the outer frame 6 of the intermediatecrystal plate 2 as illustrated in FIG. 1A.

The metal paste sealing materials are applied by publicly known methodssuch as screen printing, a dispenser, or inkjet.

FIG. 5A exemplifies the state of the metal paste sealing material 18applied onto the conductive metallic thin film 9.

As shown in the figure, the metal paste sealing material 18 is such thatmetallic particles 21 are substantially uniformly dispersed in anorganic solvent 22.

Further, resin 23 is uniformly dispersed in the organic solvent 22, andprovides the metal paste sealing material with some extent of viscosity.

Due to this viscosity, a metal paste sealing material of one embodimentof the invention has a sufficient shapability so that the material canbe applied to a pattern with high precision and the pattern can bemaintained.

For example, if the foregoing metallic particles are Au, the metal pastesealing material can be formed finely to an extent of a minimum patternwidth of about 0.05 mm and a minimum thickness of about 10 μm.

Then, the intermediate crystal plate 2 is heated at relatively lowtemperatures from about 200 to about 300° C. for a short time such asabout from 10 to about 30 minutes using publicly known means such as ahot plate, a clean oven, and a belt furnace, thereby performing aprimary sintering process.

By this primary sintering process, metallic particles of theabove-described metal paste sealing material are sintered to form aprimary sintered body.

As exemplified in FIG. 5B, in a primary sintered body 24, the organicsolvent 22 and the resin 23 evaporate from the metal paste sealingmaterial so that the metallic particles 21 adjacent to one another aremelt bonded to one another with slight gaps left therebetween to form aporous structure where the metallic particles are densely in contactwith one another.

Further, in the foregoing primary sintered body, the metallic particles21 are densified in interfaces with the metallic thin films on the lowerside, and therefore are bonded with the metallic thin films.

In some cases, the resin 23 does not completely evaporate by the primarysintering process such that part of the resin remains.

In such eases, remaining resin enters into gaps between metallicparticles 21, and therefore has no effect on forming the above-describedporous structure.

According to one embodiment of the invention, the primary sintered body24 having a Young's modulus of 9 to 16 GPa and a density of 10 to 11g/cm³ in the porous structure is the most advantageous.

It has been confirmed that the metallic particles 21 are relativelymaintained in their original spherical shapes when the temperature in aprimary sintering process is low, but sintering of the surface proceedssuch that the metallic particles 21 lose their original shapes to bemelt bonded to one another as the temperature rises.

The lower the temperature in a primary sintering process, the lower theYoung's modulus and the density of the primary sintered body 24 are,whereas the higher the temperature, the higher they are.

Since heating in a primary sintering process is at relatively lowtemperatures, its thermal stress applied to the crystal resonatorelement 5 of the intermediate crystal plate 2 can be suppressed.

Then, the upper and lower substrates 3 and 4 are placed on top of theupper and lower surfaces of the intermediate crystal plate 2 while beingaligned therewith, respectively, to bring the foregoing primary sinteredbody and the metallic thin films 14, 15, and 17 into contact with eachother, and pressure is applied uniformly to the sintered body and themetallic thin films.

Thus, a secondary sintering process has been performed.

The primary sintered body 24 having the above-described Young's modulusand density is not in its original paste state and has appropriatesoftness.

There is therefore no possibility that applying pressure causes thematerial of the body to fly to adhere to the crystal resonator element 5or to be left inside a cavity 28.

Thus, after the crystal resonator element 1 is sealed, its vibrationcharacteristics are maintained without deterioration.

The conditions of application of pressure in a secondary sinteringprocess vary in accordance with dimensions of an intermediate crystalplate and substrates, the sizes of bond surfaces to be bonded, and theamount and thickness of the used metal paste sealing material.

In the embodiment, for example, pressure from about 39 to about 176N/mm² is applied for about 10 minutes.

Further, applying pressure while heating at temperatures, e.g., fromabout 200 to about 350° C. allows the above-described secondarysintering process to be performed better.

Since this heating is also is at relatively low temperatures, itsthermal stress applied to the crystal resonator element 5 of theintermediate crystal plate 2 is suppressed.

By this secondary sintering process, the metallic particles are meltbonded more densely to completely crush the gaps of the porous structureillustrated in FIG. 5B in the primary, sintered body so that arecrystallized bond film 25 is formed as exemplified in FIG. 5C.

Regarding bond films 25 to 27, the metallic particles thereof aretightly bonded to be integrated with the metallic thin films 14, 15, and17 of the upper and lower substrates in interfaces with these thinfilms.

In this way, the intermediate crystal plate 2 and each of the upper andlower substrates 3 and 4 are bonded with high air-tightness and, in thecavity 28 defined therebetween, the crystal resonator element 5 issealed in a state of floating while being cantilevered by the baseportion 5 a.

If the secondary sintering process is performed in a vacuum atmosphereor an inactive gas atmosphere, the crystal resonator element 5 can bevacuum sealed or gas sealed.

If the primary sintered body of the intermediate crystal plate 2 and themetallic thin films of the upper and lower substrates 3 and 4, which areto be bonded, are subjected to surface activation and cleaning by aplasma treatment and an ion beam treatment that have been publiclyknown, the body and the films can be bonded better and stably.

This improves bonding reliability.

As described above, a metal paste sealing material of one embodiment ofthe invention keeps sufficient and good shapability not only during itsapplication but also after the primary and secondary sinteringprocesses, and has high air tightness after the secondary sinteringprocess due to recrystallization of the metallic particles.

This enables the sealing width in the outer frame 6 to be madesignificantly narrower than a former one.

As a result, it is possible to reduce the outside dimensions of theintermediate crystal plate 2 to reduce the entire size of the crystalunit 1.

It is also possible to increase the dimensions of the cavity 28 whilemaintaining the outer dimensions of the outer frame 6, and increase, byjust the amount of the dimension increase of the cavity 28, the outsidedimensions of the crystal resonator element 5 that can be mounted tothereby improve the characteristics of a crystal unit.

According to one embodiment of the invention, since an organic solventand resin of the foregoing metal paste sealing material sufficientlyevaporate in the primary sintering process, outer gas of the organicsolvent or the resin is not generated from a primary sintered body, andis not left in the cavity 28 of the crystal unit 1 in the secondarysintering process.

Outer gas of the organic solvent or the resin cannot be sequentiallygenerated from the bond films 25 to 27.

Therefore, good quality and frequency characteristics of the crystalunit 1 can be maintained for a long period.

In another embodiment, the foregoing metal paste sealing material canalso be applied to the metallic thin films 14, 15, and 17 of theforegoing upper and lower substrates.

A primary sintering process is performed by heating the metal pastesealing material on these metallic thin films in the same way as in thecase of the intermediate crystal plate 2, thereby forming a primarysintered body of the aforementioned porous structure.

Then, the intermediate crystal plate 2 is sandwiched between the upperand lower substrates 3 and 4 while being aligned therewith to bringtheir primary sintered bodies into contact with each other, and pressureis applied to the primary sintered bodies in the same way as in theabove-described embodiment.

Thus, a secondary sintering process has been performed.

In interfaces between the primary sintered bodies, the metallicparticles are closely melt bonded to be recrystallized so thatsubstantially one bond film is formed.

Therefore, the intermediate crystal plate 2 and the upper and lowersubstrates 3 and 4 can be more firmly bonded.

In still another embodiment, the foregoing metal paste sealing materialmay be applied only to the metallic thin films 14, 15, and 17 of theforegoing upper and lower substrates, not to the foregoing intermediatecrystal plate.

Since the aforementioned primary sintering process is performed only forthe upper and lower substrates, the heating temperature cannot affect acrystal resonator element of the intermediate crystal plate 2.

In yet another embodiment, first the intermediate crystal plate 2 andone of the upper and lower substrates 3 and 4, and then the intermediatecrystal plate 2 and the other may be bonded.

In the latter bonding, the secondary sintering process may be performedin a vacuum or predetermined gas atmosphere to vacuum seal or gas sealthe crystal resonator element 5.

Further, in performing the secondary sintering process, pressure isapplied to a crystal wafer while heating it at appropriate temperatures.

Next, a process of collectively manufacturing a large number of crystalunits 1 by applying a method of one embodiment of the invention will bedescribed.

First, as illustrated in FIG. 6A, a large-sized intermediate crystalwafer 30 is prepared in which a plurality of intermediate crystal plates2 are arranged sequentially in the longitudinal and horizontaldirections.

The shapes of the crystal resonator element 5 and the outer frame 6 ofthe each intermediate crystal plate 2 are formed, e.g., by photoetchingthe crystal wafer.

In areas corresponding to the crystal resonator element 5 and the outerframe 6 of the upper and lower surfaces of the intermediate crystalwafer 30, a conductive material is deposited by evaporation, sputtering,etc., to form a film and the film is patterned, thereby forming theaforementioned excitation electrodes, wiring films, and conductivemetallic thin films.

Concurrently, large-sized upper and lower crystal wafers 31 and 32 inwhich, respectively, a plurality of upper and lower substrates 3 and 4are arranged sequentially in the longitudinal and horizontal directionsare prepared.

In the both crystal wafers 31 and 32, a plurality of recesses 3 a and 4a are formed on the surfaces each facing the intermediate crystal wafer30, e.g., by etching or sand blasting.

Further, in the both crystal wafers 31 and 32, metallic thin films (notillustrated) corresponding to the metallic thin films 14 on lowersurfaces of the upper substrates 3 and the metallic thin films 15 and 17on the upper surfaces of the lower substrates 4 are formed on thesurfaces each facing the intermediate crystal wafer 30, e.g., byevaporation or sputtering.

In the lower crystal wafer 32, a circular through-hole 33 is formed ateach intersection of outer lines of the lower substrates 4 that areorthogonal to each other in the longitudinal and horizontal directions.

On the lower surface of the lower crystal wafer 32, a leading electrodeof each lower substrate 4 to the outside is formed.

Then, as illustrated in FIG. 6B, metal paste sealing materials 34 and 35of one embodiment of the invention are applied onto the aforementionedconductive metallic thin films in areas corresponding to the outerframes 6 on the upper and lower surfaces of the intermediate crystalwafer 30.

The metal paste sealing materials 34 and 35 have a moderate viscositydue to the above-described proportions, and therefore they can easily beapplied to a fine pattern by screen printing and the pattern can bemaintained.

In another embodiment, the materials may be continuously applied using adispenser 36 as illustrated by an imaginary line in FIG. 6B, or may beapplied in sequential dots by an inkjet method.

The metal paste sealing material of one embodiment of the invention canbe applied in dots having a diameter of 0.1 mm and a thickness of 15 μmdue to the above-described proportions.

In a still another embodiment, a resist frame is formed to define anarea to which the foregoing metal paste sealing material is to beapplied, and the inside of the area is filled with the metal pastesealing material, allowing the metal paste sealing material to beapplied.

This resist frame can be formed with high precision by patterning aphotoresist utilizing a photolithography technique, and can easily beremoved after application of the metal paste sealing material or aprimary sintering process.

Then, the intermediate crystal wafer 30 is heated at temperatures aboutfrom 200 to about 300° C. for a predetermined time by a hot plate or thelike in the same way as in the case illustrated in FIG. 1A, therebyperforming a primary sintering process.

By this primary sintering process, the metal paste sealing materials 34and 35 are sintered to form a primary sintered body of theaforementioned porous structure.

The upper and lower crystal wafers 31 and 32 are placed on top of upperand lower surfaces of the intermediate crystal wafer 30 while beingaligned therewith as illustrated in FIG. 7A, and a secondary sinteringprocess is performed by applying pressure to the wafers in the same wayas in the case illustrated in FIG. 1B.

The foregoing secondary sintering process may be performed while theforegoing crystal wafers are heated at temperatures, e.g., from about200 to about 350° C.

The secondary sintering process may be performed in a vacuum orpredetermined gas atmosphere to vacuum seal or gas seal the crystalresonator element 5.

Further, the primary sintered bodies on both upper and lower surfaces ofthe foregoing intermediate crystal wafer and metallic thin films of theupper and lower wafers are subjected to a plasma treatment by anappropriate reaction gas or uniformly subjected to surface activation byapplying ion beams.

This allows achievement of better and more stable bonding andimprovement in bonding reliability.

In this way, a crystal wafer laminate 37 illustrated in FIG. 7B isobtained in which the intermediate crystal wafer 30 and the upper andlower crystal wafers 31 and 32 are hermetically bonded.

As illustrated in the figure, the crystal wafer laminate 37 is cut alongouter lines 38 of crystal units that are orthogonal in the longitudinaland horizontal directions by dicing, etc., to be divided into individualelements, thereby completing the crystal unit 1 illustrated in FIGS. 1Aand 1B.

The aforementioned leading electrode on the bottom surface of each lowersubstrate 4 may also be formed by sputtering, etc., while the crystalwafer laminate 37 is in its state before being diced.

As illustrated in FIG. 8, a metal paste sealing material of oneembodiment of the invention was applied onto the intermediate crystalwafer 30 illustrated in FIG. 6A by screen printing.

A metal paste sealing material 39 was formed in a fine, rectangularframe-shaped pattern in each outer frame 6 such that constant widthareas 30 a were left on both sides of the outer lines 38 for dicing.

The composition and solid state properties of the metal paste sealingmaterial 39 were set as follows.

Composition: metal (Au) particle content: 88 wt %, average particle sizeof 0.3 μm

-   -   organic solvent content: 10.4 wt %    -   resin content: 1.6 wt %        Solid state properties: viscosity: 190 Pa·s    -   thixotropic ratio: 4.9

As a result, a fine pattern of the metal paste sealing material 39 wasformed in a rectangular frame shape having external dimensions of 2.95mm (L1)×1.25 mm (W1), internal dimensions of 2.85 mm (L2)×1.15 mm (W2),and a line width of 0.05 mm.

In another embodiment, first the intermediate crystal wafer 30 and oneof the upper and lower crystal wafers 31 and 32, and then theintermediate crystal wafer 30 and the other may be bonded.

In this case, in a later process of bonding a third one of the foregoingcrystal wafers, a secondary sintering process may be performed in avacuum or predetermined gas atmosphere to vacuum seal or gas seal eachcrystal resonator element 5.

With two of the foregoing crystal wafers bonded in advance,characteristics testing and frequency measurement may be carried out foreach crystal resonator element 5.

Further excitation electrodes of each crystal unit 5 may be partiallyremoved, e.g., by irradiation of laser light so that the frequency isindividually adjusted.

In this case, in the intermediate crystal wafer 30, the aforementionedconductive metallic thin film is formed to obtain a pattern on each ofthe upper and lower surfaces of each outer frame 6 except for portionsof line widths for dicing along the outer lines 38 of crystal units.

Similarly, in the upper and lower crystal wafers 31 and 32, theaforementioned metallic thin films corresponding to the metallic thinfilms 14, 15, and 17 of the aforementioned upper and lower substratesare formed to obtain patterns except for portions of line widths fordicing along the outer lines 38 of crystal units.

The metal paste sealing materials 34 and 35 have good shapability asdescribed above and are applied only onto the conductive metallic thinfilm.

Each crystal resonator element 5 of the intermediate crystal wafer 30 istherefore electrically separate and independent from other adjacentcrystal resonator elements in the wafer state around the primary andsecondary sintering processes.

The invention can be similarly applied to piezoelectric devices ofvarious package structures that are different from the above-mentionedfirst embodiment.

FIGS. 9A and 9B illustrate a process of sealing a crystal unit of asecond embodiment by a method of one embodiment of the invention.

A crystal unit 41 of the present embodiment has a package structureincluding a rectangular box-shaped base 42 and a flat plate-shaped lid43, which are made of insulating materials.

The base 42 is formed in a box shape having an open top by laminating aplurality of ceramic thin plates 42 a to 42 c, as illustrated in FIGS.9A and 10.

A crystal resonator element 45 is mounted in a cavity 44 defined insidethe base 42.

The crystal resonator element 45 of the embodiment is a thickness shearmode resonator element made, e.g., of an AT cut crystal plate, andexcitation electrodes 46 are formed on both upper and lower surfaces ofthe element and leading electrodes 47 from the excitation electrodes areformed on both sides of one end of the element, that is, a base portion45 a.

A pair of electrode pads 48 are formed near one end in the longitudinaldirection on the bottom surface of the cavity 44 of the base 42.

The leading electrodes 47 are fixed to the electrode pads 48corresponding thereto with a conductive adhesive 49 in the base portion45 a, so that crystal resonator element 45 is electrically coupled andsubstantially horizontally supported while being cantilevered.

As illustrated in FIG. 11, the lid 43 of the embodiment is formed of aflat, rectangular thin plate 50 of a glass material.

The entire lower surface of the lid 43 is coated with a metallic thinfilm 51.

The metallic thin film 51 may be formed only in an area 43 a, which isto be bonded with an top end surface of the base 42, along the peripheryof the lower surface of the lid 43 as illustrated by an imaginary linein FIG. 11.

In another embodiment, the lid 43 may be formed of a thin plate of aninsulating material, such as quartz crystal and ceramics, and a metallicmaterial, such as Kovar and SUS (stainless steel).

If the lid 43 is made of a metallic material, its metallic surfaceitself may be used as a surface to be bonded without use of the metallicthin film 51 depending on the quality of material.

A metallic thin film 52 is adhered to a top end surface of the base 42.

The metallic thin films 51 and 52 of the lid 43 and the base 42 arepreferably formed by laminating an Au film on a base film made, e.g., ofa Cr film, a Ni film, a Ni/Cr film, a Ti film, or a Ni—Cr film by amethod such as sputtering, evaporation, or plating in the same way asfor the upper and lower substrates 3 and 4 of the first embodiment.

Alternatively, the metallic thin films 51 and 52 may be formed of Al,Ag, Cu, Pd, Pt, Sn, Ti, Al/Si, or Ni/Cr.

According to a method of one embodiment of the invention, a metal pastesealing material 53 of one embodiment of the invention, which has beendescribed above, is applied onto a metallic thin film 52 on the top endsurface of the base 42 as illustrated in FIG. 9A.

The foregoing metal paste sealing material is applied in the same way bya method such as screen printing, a dispenser, or inkjet.

Then, a primary sintering process is performed by heating the base 42 atrelatively low temperatures from about 200 to about 300° C. in the sameway as in the first embodiment.

This sinters the metallic particles of the metal paste sealing material53 to form a primary sintered body having the aforementioned porousstructure.

Since heating in the primary sintering process is at relatively lowtemperatures, its thermal stress applied to the crystal resonatorelement 45 in the base 42 is suppressed.

In this state, characteristics testing and frequency measurement of thecrystal resonator element 45 may be carried out, and further theexcitation electrode 46 may be partially removed, e.g., by irradiationof laser light so that the frequency is adjusted.

Applying the metal paste sealing material 53 and the primary sinteringprocess may be performed either before or after mounting the crystalresonator element 45 on the base 42.

Then, the lid 43 is placed on the top end surface of the base 42 whilebeing aligned therewith, and pressure is applied under the sameconditions as in the case of the first embodiment, thereby performing asecondary sintering process.

The foregoing primary sintered body has appropriate softness asdescribed above, and therefore there is no possibility that applyingpressure causes the material of the body to fly to adhere to the crystalresonator element 45 or to be left inside the cavity 44.

Thus, after the crystal resonator element 41 is sealed, its vibrationcharacteristics are maintained without being deteriorated.

As illustrated in FIG. 9B, in the foregoing primary sintered body, theforegoing metallic particles are more densely melt bonded to form arecrystallized bond film 54 by the secondary sintering process.

In the bond film 54, the metallic particles are tightly bonded to themetallic thin film 51 of the lid to be integrated with each other in aninterface of the metallic particles and the metallic thin film 51.

Thus, the base 42 and the lid 43 are bonded together with high airtightness.

In the bond film 54 where the foregoing metallic particles arerecrystallized, there is no possibility that the film is remelted in ahigh temperature environment as in a bonding portion produced by aformer brazing metal material.

This therefore ensures high air tightness over time.

Accordingly, if the crystal unit 41 is heated in a repair process of afaulty part after being mounted on a circuit board or the like, airtightness cannot be reduced, allowing high reliability to be maintained.

Further, applying pressure while heating at temperatures, e.g., fromabout 200 to about 350° C. allows the foregoing secondary sinteringprocess to be performed better.

Since this heating is also at relatively low temperatures, its thermalstress applied to the crystal resonator element 45 is suppressed.

The secondary sintering process may be performed in a vacuum or desiredgas atmosphere to vacuum seal or gas seal the crystal resonator element45.

In the embodiment, a metal paste sealing material of one embodiment ofthe invention keeps sufficient and good shapability not only during itsapplication but also after the primary and secondary sinteringprocesses, and has high air tightness due to recrystallization of themetallic particles after the secondary sintering process.

This enables the sealing width on the top end surface of the base 42 tobe made significantly narrower than a former one.

As a result, it is possible to reduce the outside dimensions of the base42 to reduce the entire size of the crystal unit 1.

It is also possible to increase the dimensions of the cavity 44 whilemaintaining the outer dimensions of the base 42, and increase, by justthe amount of the dimension increase of the cavity 44, the outsidedimensions of the crystal resonator element 45 that can be mounted,thereby improving the characteristics of a crystal unit.

Similarly, a metal paste sealing material of one embodiment of theinvention is applied not only to the top end surface of the base 42 butalso to the metallic thin film 51 of the lid 43 to form a primarysintered body of the aforementioned porous structure by a primarysintering process.

This primary sintered body is brought into contact with theaforementioned primary sintered body on the top end surface of the base42.

In this way, a secondary sintering process may be performed.

Alternatively, a metal paste sealing material of one embodiment of theinvention is applied only to the metallic thin film 51 on the lid 43 toform a primary sintered body of the porous structure by the primarysintering process.

This primary sintered body is brought into contact directly with themetallic thin film 52 on the top end surface of the base 42.

In this way, the secondary sintering process may be performed.

In this case, since the foregoing primary sintering process is performedonly for the lid 43, heating temperature cannot affect the crystalresonator element 45 mounted on the base 42.

The invention can be similarly applied to piezoelectric devices havingpackage structures that are different from the above-mentioned first andsecond embodiments.

FIG. 12 illustrates a crystal oscillator 61 having a similar packagestructure to the second embodiment.

The crystal oscillator 61 is formed in a box shape having an open top bylaminating a plurality of ceramic thin plates, like the base 42 in thesecond embodiment.

A crystal resonator element 65 is mounted in a cavity 64 defined insidethe crystal oscillator 61.

The crystal resonator element 65 is a thickness shear mode resonatorelement made of an AT cut crystal plate, and excitation electrodes 66are formed on both upper and lower surfaces of the element and leadingelectrodes 67 from the excitation electrodes are formed on both sides ofone end, that is, a base portion 65 a.

A pair of electrode pads 68 are formed near one end in the longitudinaldirection on the bottom surface of a cavity 64 of a base 62.

The crystal resonator element 65 is electrically coupled andsubstantially horizontally supported while being cantilevered by thebase portion 65 a by fixing the leading electrodes to the electrode pads68 corresponding thereto with a conductive adhesive 69.

As illustrated in FIG. 13, a metallic thin film 70 having apredetermined width adheres to the top end surface of the base 62 alongthe outer periphery.

Further, bonding pads 71 and 72 are formed inside the metallic thin film70 to be separate therefrom on the top end surface in both ends in thelongitudinal direction of the base 62.

The foregoing each bonding pad is used as a terminal coupled to theaforementioned excitation electrode of the crystal resonator element 65or an outside power supply, a circuit, etc., through wiring, which isnot illustrated.

A lid 63 of the present embodiment is formed of a flat, rectangular thinplate that is made of a silicon material.

As illustrated in FIG. 14, a metallic coating film 73 having apredetermined width adheres to an area, which is to be bonded with thetop end surface of the base 62, along the outer periphery of the lowersurface of the lid 63. Inside the metallic coating film 73, the bondingpads 74 and 75 are disposed, as terminals directly coupled to thebonding pads 71 and 72 on the top end surface of the base 62,respectively, at positions corresponding thereto in both ends in thelongitudinal direction of the lid 63.

Further, in an area inside the metallic coating film 73 of the lid 63,an integrated circuit for controlling the drive of the crystal resonatorelement 65 and wiring coupling the integrated circuit with the bondingpads 74 and 75 are formed (not illustrated).

The base 62 and the lid 63 are bonded utilizing a method of oneembodiment of the invention, as illustrated in FIGS. 15A and 15B.

In the same way as in the case of the second embodiment, a metal pastesealing material 76 of one embodiment of the invention is applied ontothe metallic thin film 70 on the top end surface of the base 62 by amethod such as the above-described screen printing.

A primary sintering process is performed by heating the base 62 atrelatively low temperatures from about 200 to about 300° C., causing themetal paste sealing material 76 to become a primary sintered body of aporous structure.

On the other hand, on each bonding pad of the base 62, e.g., an Au ball78 is deposited to form a bump as illustrated in FIG. 15A.

For the bump, various publicly known conductive materials, such assolder, other than Au may be used.

The bump may be formed on each bonding pad of the lid 63 instead of thebonding pad of the base 62.

Then, as illustrated in FIG. 15B, the lid 63 is placed on the top endsurface of the base 62 while being aligned therewith, and applyingpressure while heating in the same way as in the cases of theabove-mentioned embodiments, thereby performing a secondary sinteringprocess.

Accordingly, in the foregoing primary sintered body of the metal pastesealing material 76, metallic particles contained therein are denselymelt bonded to be recrystallized.

The recrystallized metallic particles are integrated with the metallicthin film 70 of the base 62 and the metallic coating film 73 of the lid63 to form a bond film 77.

At the same time, each bump 79 is deposited to each bonding pad of thelid 63 by effects caused by heating and applying pressure in thesecondary sintering process.

Thus, each one of the bonding pads 71 and 72 of the base 62 areelectrically coupled to each corresponding one of the bonding pads 74and 75 of the lid 63.

The bonding pads of the base 62 and the bonding pads of the lid 63 canbe electrically coupled to each other using various publicly knownconductive coupling materials other than bumps of metal such as Au.

For example, conductive adhesives, conductive pastes, and metallicpastes can be used.

By means of the bond film 77, the base 62 and the lid 63 are bondedtogether with high air tightness so that the crystal resonator element65 is vacuum sealed or gas sealed inside a package.

Further, according to the embodiment, the lid 63 functioning as a driveIC chip of the crystal resonator element 65 is hermetically bonded withthe base 62 on which the crystal resonator element 65 is mounted.

This allows achievement of a smaller, lower surface mount crystaloscillator.

A crystal oscillator into which an IC chip to drive a crystal resonatorelement is integrated in this way can be applied to a package structurein the first embodiment.

FIG. 16 illustrates a crystal oscillator 81 of such a package structureas in the first embodiment.

In the crystal oscillator 81, an upper substrate 83 serving as a coverof the package and a lower substrate 84 serving as a base are laminatedon and under an intermediate crystal plate 82 having a crystal resonatorelement so that the substrates and the intermediate crystal plate areintegrally bonded in the same way as for the crystal unit 1 in the firstembodiment.

In the embodiment, the intermediate crystal plate 82 is formed of an ATcut crystal plate having a uniform thickness and the lower substrate 84is formed of a crystal thin plate and a glass material or silicon, etc.,whereas the upper substrate 83 is formed of a silicon material.

The intermediate crystal plate 82 includes a thickness shear modecrystal resonator element 85 and an outer flame 86 integrally connectedthereto with a base portion 85 a as illustrated in FIG. 17.

Formed on the upper and lower surfaces of the crystal resonator element85 are excitation electrodes 87.

The excitation electrodes 87 are formed on the upper and lower surfacesof crystal resonator element 85, and each of wiring films 87 a is ledout from the base portion 85 a to an end in the longitudinal directionof the outer frame 86 on a side where the crystal resonator element 85is connected.

On the top surface of the outer frame 86, a conductive metallic thinfilm 88 having a predetermined width is formed along the outerperiphery.

Although the conductive metallic thin film 88 and the wiring film 87 aare separate in the embodiment as illustrated in the figure, they may beelectrically coupled.

The lower surface of the intermediate crystal plate 82 is formed in thesame way as the intermediate crystal plate 2 in FIG. 2.

A conductive metallic thin film is formed over all the periphery of thelower surface of the outer frame 86, and is electrically coupled to theforegoing wiring film led out from an excitation electrode of the lowersurface of the crystal resonator element 85.

On an end in the longitudinal direction, on a side where the crystalresonator element 85 is connected, of the lower surface of the outerframe 86, a conductive metallic thin film separated from the foregoingwiring film is formed and is electrically coupled through a through-hole89 to the wiring film 87 a on the upper surface of the outer frame 86.

Further, on the upper surface at both ends in the longitudinal directionof the outer frame 86, bonding pads 90 and 91 are formed inside theconductive metallic thin film 88 and apart therefrom.

The bonding pads 90 and 91 are used as terminals coupled to theforegoing excitation electrodes of the crystal resonator element 85 oran outside power supply, a circuit, etc., through wiring, which is notillustrated.

In the upper substrate 83, a recess 83 a is formed in a surface facingthe intermediate crystal plate 82, that is, the lower surface asillustrated in FIG. 18.

The periphery surrounding the recess 83 a in the lower surface of theupper substrate 83 constitutes a surface to be bonded with theintermediate crystal plate 82, and is coated with a metallic thin film92 that corresponds to the conductive metallic thin film 88 of theforegoing outer frame.

Inside the metallic coating film 92, bonding pads 93 and 94 aredisposed, as terminals directly coupled to the bonding pads 90 and 91 onthe tipper surface of the outer frame 86 of the intermediate crystalplate 82, at positions corresponding to the bonding pads 90 and 91 inboth ends in the longitudinal direction of the upper substrate 83,respectively.

Further, in an area inside the metallic coating film 92 of the uppersubstrate 83, an integrated circuit for controlling the drive of thecrystal resonator element 85 and wiring coupling the integrated circuitwith the bonding pads 93 and 94 are formed (not illustrated).

The intermediate crystal plate 82 and the upper and lower substrates 83and 84 are bonded utilizing a method of one embodiment of the invention.

In the same way as in the case of the first embodiment, a metal pastesealing material of one embodiment of the invention is applied onto theforegoing conductive metallic thin films on the upper and lower surfacesof the outer frame 86 of the intermediate crystal plate 82.

A primary sintering process is performed by heating the intermediatecrystal plate 82 at relatively low temperatures from about 200 to about300° C., causing the foregoing metal paste sealing material to become aprimary sintered body of a porous structure.

On each bonding pad on the upper surface of the outer frame 86 of theintermediate crystal plate 82, a bump made of various publicly knownconductive materials such as an Au ball is formed in the same manner asillustrated in FIG. 15A.

The bump may be formed on each of the foregoing bonding pads on thetipper substrate 83 instead of the intermediate crystal plate 82.

Then, the upper and lower substrates 83 and 84 are placed on top of theupper and lower surfaces of the intermediate crystal plate 82 whilebeing aligned therewith, respectively, and a secondary sintering processis performed by applying pressure while heating in the same way as inthe case of the first embodiment.

Accordingly, in the primary sintered body of the foregoing metal pastesealing material, metallic particles contained therein are densely meltbonded to be recrystallized, forming bond films 95 and 96.

At the same time, bumps 97 and 98 are deposited by effects caused byheating and applying pressure in the secondary sintering process.

Thus, each one of the bonding pads 90 and 91 of the intermediate crystalplate 82 is electrically coupled to each corresponding one of thebonding pads 93 and 94 of the upper substrate 83.

Further, the invention can be applied to quartz crystal devices such ascrystal units and crystal oscillators having tuning fork crystalresonator elements.

FIG. 19 illustrates a tuning fork crystal unit 101 having a packagestructure of the first embodiment illustrated in FIG. 1.

In the crystal unit 101, an upper substrate 103 serving as a cover ofthe package and a lower substrate 104 serving as a base are laminated onand under a flat plate-shaped intermediate crystal plate 102 having acrystal resonator element so that the substrates and the intermediatecrystal plate are integrally bonded in the same way as for the crystalunit 1 in the first embodiment.

The intermediate crystal plate 102 is formed of an AT cut crystal platehaving a uniform thickness, and the upper and lower substrates 103 and104 are preferably formed of a crystal thin plate or a glass material,silicon, and the like.

The intermediate crystal plate 102 includes a tuning fork crystalresonator element 105 having a pair of resonating arms and an outerframe 106 integrally connected thereto with a base portion 105 a asillustrated in FIGS. 20A and 20B.

One excitation electrode 107 formed on a surface of the foregoingresonating arm is led out from the base portion 105 a, and iselectrically coupled to a conductive metallic thin film 108 on the uppersurface of the outer frame 106.

The other excitation electrode 109 formed on the surface of theforegoing resonating arm is led out from the foregoing base portion inthe same way, and is electrically coupled to a conductive metallic thinfilm 110 on the lower surface of the outer frame 106.

In an end, on a side where the crystal resonator element 105 isconnected, in the longitudinal direction of the outer frame 106, aconductive metallic thin film 112 that is separate from the conductivemetallic thin film 110 on the lower surface of the outer frame 106 iselectrically coupled to the conductive metallic thin film 108 on theupper surface of the outer frame 106 tough a conductive film inside athrough hole 111.

In the upper substrate 103, a recess 103 a is formed in a surface facingthe intermediate crystal plate 102, and a metallic thin film 113 isformed by the periphery surrounding the recess, that is, on a surface tobe bonded with the intermediate crystal plate 102, as illustrated inFIG. 21.

In the lower substrate 104, a recess 104 a is formed on a surface facingthe intermediate crystal plate 102, and metallic thin films 114 and 115corresponding to the conductive metallic thin films 110 and 112 on thelower surface of the intermediate crystal plate by the peripherysurrounding the recess, that is, on a surface to be bonded with theintermediate crystal plate 102, as illustrated in FIG. 22.

The crystal resonator element 105 is held and contained in a cavitydefined by these recesses while being cantilevered by the base portion105 a.

Further, a sealing hole 116 is provided in and through the lowersubstrate 104 substantially at the center thereof.

The intermediate crystal plate 102 and the upper and lower substrates103 and 104 are bonded utilizing a method of one embodiment of theinvention.

In the same way as in the case of the first embodiment, a metal pastesealing material of one embodiment of the invention is applied onto theconductive metallic thin films 108, 110, and 112 on the lower surface ofthe outer frame 106 of the intermediate crystal plate 102.

A primary sintering process is performed by heating the intermediatecrystal plate 102 at relatively low temperatures from about 200 to about300° C., causing the foregoing metal paste sealing material to become aprimary sintered body of a porous structure.

Then, the upper and lower substrates 103 and 104 are placed on top ofthe upper and lower surfaces of the intermediate crystal plate 102 whilebeing aligned therewith, respectively, and a secondary sintering processis performed by applying pressure while heating in the same way as inthe case of the first embodiment.

Accordingly, in the primary sintered body of the foregoing metal pastesealing material, metallic particles contained therein are densely meltbonded to be recrystallized.

This results in forming bond films 117 to 119 between the upper andlower surfaces of the intermediate crystal plate 102 and the upper andlower substrates 103 and 104, respectively, such that the crystalresonator element 115 is sealed in a package.

Then, the package is placed in a vacuum atmosphere, and the sealing hole116 is hermetically closed with a sealant 120.

As the sealant 120, a low melting metallic material such as Au—Sn may beused.

The metallic material is placed in a sealing hole and is irradiated withlaser light from the outside so that metallic material is deposited intothe sealing hole.

It is desirable to coat the inner surface of the sealing hole in advancewith a metallic film because this allows better deposition of a metallicmaterial.

In this way, in the present embodiment, unnecessary gas, which can begenerated from the foregoing metal paste sealing material duringhermetic sealing of a package, can be excluded from the inside of thepackage using the foregoing sealing hole.

The crystal unit 101 can thus be sealed with a higher degree of vacuum.

Although preferred embodiments of the invention have been described indetail, it is to be understood that the invention may be practiced byadding various changes and modifications to the foregoing embodiments inits technical scope, as apparent to those skilled in the art.

For example, the aforementioned intermediate crystal plate may be formedof various publicly known piezoelectric materials such as lithiumtantalite and lithium niobate other than quartz crystal.

The invention may also be applied to piezoelectric devices such asresonators, filters, and sensors other than crystal units oroscillators.

IC elements and the like may be mounted in a package in the samestructure as in the second embodiment.

A piezoelectric vibrating gyro element instead of a crystal resonatorelement may also be mounted to constitute a piezoelectric vibrating gyrosensor.

Further, the invention may be applied so as to hermetically seal variouselectronic components other than piezoelectric devices in packages.

In this case, components constituting a package have metallic surfacesas surfaces to be bonded with each other.

A metal paste sealing material of one embodiment of the invention isapplied to at least one metallic surface, and a primary sinteringprocess is performed by heating the surface in the same way as each ofthe aforementioned embodiments.

After a primary sintered body having the aforementioned porous structureis formed, both components are put together while one metallic surfacehaving the primary sintered body formed thereon is brought into contactwith the other metallic surface.

Metallic particles contained in the primary sintered body are denselyrecrystallized by applying pressure to the primary sintered body whileoptionally heating it, thereby performing a secondary sintering process.

Thus, the package can be hermetically bonded to hermetically sealelectronic components inside the package.

1. A method for sealing a quartz crystal device, in order to bond an upper substrate and a lower substrate with upper and lower surfaces, respectively, of an intermediate crystal plate in which a crystal resonator element and an outer frame are integrally connected to hermetically seal the crystal resonator element in a cavity defined between the upper substrate and the lower substrate, the method comprising: (a) applying a metallic thin film on each of upper and lower surfaces of the outer frame of the intermediate crystal plate, and a metallic thin film on a surface to be bonded with the outer frame of each of the upper substrate and the lower substrate; (b) applying a metal paste sealing material including an organic solvent and metallic particles to at least one of the metallic thin film on the upper surface of the outer frame and the metallic thin film of the upper substrate, and heating the metal paste sealing material to evaporate substantially all the organic solvent to form a primary sintered body having a porous structure with a Young's modulus of 9 to 16 GPa and a density of 10 to 17 g/cm³, wherein the metallic particles of the metal paste sealing material each have an average particle size from 0.1 to 1.0 μm, and the metal paste sealing material comprises the metallic particles in a proportion of from 88 to 93 percent by weight of the metal paste sealing material, the organic solvent in a proportion of from 5 to 15 percent by weight of the metal paste sealing material, and a resin material in a proportion of from 0.01 to 4.0 percent by weight of the metal paste sealing material, to hermetically seal the crystal resonator element; (c) applying the metal paste sealing material to at least one of the metallic thin film on the lower surface of the outer frame and the metallic thin film of the lower substrate, and heating the metal paste sealing material to evaporate substantially all the organic solvent to form a primary sintered body having a porous structure with a Young's modulus of 9 to 16 GPa and a density of 10 to 17 g/cm³; (d) placing the intermediate crystal plate and the upper substrate adjacent to each other while bringing one of the metallic thin films having the primary sintered bodies formed thereon into contact with the other of the metallic thin films, and applying pressure to the primary sintered bodies to densely recrystallize the metallic particles thereof to hermetically bond the intermediate crystal plate and the upper substrate in the outer frame; and (e) placing the intermediate crystal plate and the lower substrate adjacent to each other while bringing one of the metallic thin films having the primary sintered bodies formed thereon into contact with the other of the metallic thin films, and applying pressure to the primary sintered bodies to densely recrystallize the metallic particles thereof to hermetically bond the intermediate crystal plate and the lower substrate in the outer frame.
 2. The method for sealing a quartz crystal device according to claim 1, the upper substrate and the lower substrate being made of quartz crystal.
 3. The method for sealing a quartz crystal device according to claim 1, the upper substrate being made of a silicon material and including an integrated circuit that drives the crystal resonator element and a terminal coupled to the integrated circuit; the intermediate crystal plate having a terminal to be coupled to the terminal of the upper substrate on the upper surface of the outer frame; and in the step (d), in placing the intermediate crystal plate and the upper substrate on top of each other, the terminal of the upper substrate being directly coupled with the terminal of the intermediate crystal plate with a conductive coupling material.
 4. The method for sealing a quartz crystal device according to claim 1, the steps (d) and (e) being such that heating is simultaneous with applying pressure.
 5. The method for sealing a quartz crystal device according to claim 1, the heating of the metal paste sealing material being carried out at a temperature between 200° C. and 350° C., inclusive.
 6. The method for sealing a quartz crystal device according to claim 1, the metal paste sealing material being heated for a time period between 10 to 30 minutes, inclusive.
 7. The method for sealing a quartz crystal device according to claim 1, the metallic particles of the metal paste sealing material being made of at least one of: Au, Ag, Pt, and Pd. 